Write a linter rule for $this reassignment

Summary: Re-assigning `$this` is an invalid PHP construct, see https://3v4l.org/TauX9.

Test Plan: Added unit tests.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14514

src/__phutil_library_map__.php

@@ -319,6 +319,8 @@ phutil_register_library_map(array(
     'ArcanistTestXHPASTLintSwitchHook' => 'lint/linter/__tests__/ArcanistTestXHPASTLintSwitchHook.php',
     'ArcanistTextLinter' => 'lint/linter/ArcanistTextLinter.php',
     'ArcanistTextLinterTestCase' => 'lint/linter/__tests__/ArcanistTextLinterTestCase.php',
+    'ArcanistThisReassignmentXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistThisReassignmentXHPASTLinterRule.php',
+    'ArcanistThisReassignmentXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistThisReassignmentXHPASTLinterRuleTestCase.php',
     'ArcanistTimeWorkflow' => 'workflow/ArcanistTimeWorkflow.php',
     'ArcanistToStringExceptionXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistToStringExceptionXHPASTLinterRule.php',
     'ArcanistToStringExceptionXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistToStringExceptionXHPASTLinterRuleTestCase.php',
@@ -695,6 +697,8 @@ phutil_register_library_map(array(
     'ArcanistTestXHPASTLintSwitchHook' => 'ArcanistXHPASTLintSwitchHook',
     'ArcanistTextLinter' => 'ArcanistLinter',
     'ArcanistTextLinterTestCase' => 'ArcanistLinterTestCase',
+    'ArcanistThisReassignmentXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
+    'ArcanistThisReassignmentXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
     'ArcanistTimeWorkflow' => 'ArcanistPhrequentWorkflow',
     'ArcanistToStringExceptionXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
     'ArcanistToStringExceptionXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',

src/lint/linter/xhpast/rules/ArcanistThisReassignmentXHPASTLinterRule.php

@@ -0,0 +1,39 @@
+<?php
+
+final class ArcanistThisReassignmentXHPASTLinterRule
+  extends ArcanistXHPASTLinterRule {
+
+  const ID = 91;
+
+  public function getLintName() {
+    return pht('`%s` Reassignment', '$this');
+  }
+
+  public function process(XHPASTNode $root) {
+    $binary_expressions = $root->selectDescendantsOfType('n_BINARY_EXPRESSION');
+
+    foreach ($binary_expressions as $binary_expression) {
+      $operator = $binary_expression->getChildOfType(1, 'n_OPERATOR');
+
+      if ($operator->getConcreteString() != '=') {
+        continue;
+      }
+
+      $variable = $binary_expression->getChildByIndex(0);
+
+      if ($variable->getTypeName() != 'n_VARIABLE') {
+        continue;
+      }
+
+      if ($variable->getConcreteString() == '$this') {
+        $this->raiseLintAtNode(
+          $binary_expression,
+          pht(
+            '`%s` cannot be re-assigned. '.
+            'This construct will cause a PHP fatal error.',
+            '$this'));
+      }
+    }
+  }
+
+}

src/lint/linter/xhpast/rules/__tests__/ArcanistThisReassignmentXHPASTLinterRuleTestCase.php

@@ -0,0 +1,10 @@
+<?php
+
+final class ArcanistThisReassignmentXHPASTLinterRuleTestCase
+  extends ArcanistXHPASTLinterRuleTestCase {
+
+  public function testLinter() {
+    $this->executeTestsInDirectory(dirname(__FILE__).'/this-reassignment/');
+  }
+
+}

src/lint/linter/xhpast/rules/__tests__/this-reassignment/class.lint-test

@@ -0,0 +1,8 @@
+<?php
+class SomeClass {
+  public function __construct() {
+    $this = null;
+  }
+}
+~~~~~~~~~~
+error:4:5

src/lint/linter/xhpast/rules/__tests__/this-reassignment/this.lint-test

@@ -0,0 +1,4 @@
+<?php
+$this = null;
+~~~~~~~~~~
+error:2:1